A method for synchronizing local timing generators of an automation system is known from DE 4215 380 A1. With this method a local timing generator is synchronized by means of time information which is derived from the time information of a central timing generator plus a correction value corresponding to the transmission and processing time. The time information is transmitted by a transmission unit associated with the central timing generator only if it deviates from the current time by less than a predetermined amount.
DE 199 173 54 A1 discloses a synchronization method for a master unit and at least one slave unit with internal timing generators that are to be synchronized with one another. With this method time signals are transmitted by the master unit to the slave unit via two communication paths, which signals require propagation times on the two communication paths to the slave unit. The difference in the propagation times of the time signals is recorded. The timing generators are then synchronized taking the propagation times into account.
DE 197 03 963 A1 discloses a method for exchanging data between electronic modules which can be arranged on a decentralized basis. With this method one module is used as a clock generator and, what is more, without a redundancy mechanism.
Various standardized communication systems, also referred to as bus systems, which support data exchange between two or more electronic modules or devices are known from the prior art, in particular also for use in automation systems. Examples of such communication systems are: Fieldbus, Profibus, Ethernet, Industrial Ethernet, FireWire, or also PC-internal bus systems (PCI). These bus systems are in each case designed or optimized for different fields of application and permit a decentralized control system to be set up. Very fast and reliable communication systems with predictable response times are required for process control and monitoring in automated production and in particular in digital drive technologies.
With parallel bus systems, such as SMP, ISA, PCI or VME for example, a very fast and simple means of communication can be set up between different modules. These known bus systems are therefore deployed in particular in computers and PCs.
Synchronous, clock-controlled communication systems with equidistance properties are known in particular from automation technology. By this is meant a system comprising at least two subscribers that are interconnected via a data network for the purpose of reciprocal exchange of data or reciprocal transmission of data. In this case the data exchange takes place cyclically in equidistant communication cycles which are predetermined by the communication clock timing used by the system. Subscribers are for example central automation devices, programming, planning or control devices, peripheral devices such as, for example, input/output modules, drives, actuators, sensors, stored program controls (SPC) or other control units, computers, or machines which exchange electronic data with other machines, and in particular process data from other machines. In the following, control units are understood to mean regulating or controlling units of any kind.
An equidistant deterministic cyclical data exchange in communication systems is based on a common clock or time base for all the components involved in the communication. The clock or time base is transmitted to the other components by a specially designated component (clock master or timer). With isochronous realtime Ethernet, the clock or time base is specified by a clock master by the sending of synchronization telegrams.